A variety of screw and plate type fastener systems have been designed for securing successive layers of roof covering materials (e.g., a water impervious membrane overlying a layer of insulation) to rooftops made of steel, gypsum, tectum, or wood. Such systems generally include an elongated screw which penetrates a flat plate (washer). The plate (sometimes referred to as a stress plate) clamps down the roof covering membrane and/or insulation when the screw is tightened in the rooftop and tends to prevent the membrane or insulation from pulling vertically over the head of the screw.
Screw and plate fasteners are most frequently used on commercial buildings having a flat rooftop. In general, a layer of insulation is laid out overlying the roof deck and fastened down with screw and plate fasteners. A sheet of membrane, typically marketed in rolls, is then laid out over the insulation. Fasteners are then installed along the edges of the membrane sheet. The distance from the edge and the distance between fasteners is determined in accordance with the deck and membrane material and anticipated conditions. More specifically, a minimum force which will cause the membrane to tear away from the fastener (or the fastener to pull out of the roof), is prescribed. The number of fasteners per unit linear distance is chosen to ensure that the membrane and/or insulation will be retained.
To install the respective fastener, a pilot hole is, if needed, drilled at the desired location through the roof membrane, insulation, and deck. A plastic or metal screw is placed through a retaining stress plate and then installed in the hole, engaging the roof deck so that the plate is held tightly against the membrane. After the fasteners have been installed along the edge of the first sheet, a successive sheet of membrane is laid out, with one edge overlapping the edge of the first sheet of membrane covering the fasteners. That edge of the successive sheet is bonded (e.g., chemically or by heating) to the preceding sheet. The other edge of the succeeding sheet is fixed to the roof deck by fasteners in the manner previously described. Thus, the roof is covered by overlapping sheets of membrane. The membrane edges along the edges of the roof are fixed by battens or other conventional techniques.
Problems have been encountered with conventional screw and plate fasteners when employed in such rooftop environments. Wind blowing over the membrane tends to create a negative air pressure, which in turn tends to cause the membrane to pull laterally out from the fastener. To mitigate against the membrane tearing out from under the anchor due to such lateral forces, downwardly directed cleats, lugs, spikes, ribs or other protrusions on the underside of the stress plate, engage the membrane when the screw is tightened in the rooftop. See, for example, Murphy U.S. Pat. No. 4,787,188, issued Nov. 29, 1988 and Reinwall et al. U.S. Pat. No. 4,726,164, issued Feb. 23, 1989. However, over time wind vibrations and/or vibrations from within the building cause the screw to back out (unscrew) from the rooftop. These conditions can cause the head of the screw to pop up, i.e., protrude from the surrounding roofing material, which in turn leads to damage to the underlying membrane, insulation and roof.
Back out can be prevented by preventing the screw from turning (in a reverse direction) relative to the roof. This could be accomplished with a broad headed screw having lugs, spikes, ribs or the like on the underside of the head to engage the membrane and prevent counter-rotation. However, rotation of the lugs, etc., during installation would tend to tear or otherwise damage the membrane. Accordingly, fastener systems have been proposed including a plate with anti-rotation structure which engages the membrane, e.g., spikes (which also militate against lateral pull out of the membrane), a separate screw, and a mechanism to prevent counter-rotation of the screw relative to the plate.
Such a system is described in the aforementioned Dewey U.S. Pat. No. 4,380,413. Projecting pawls on the head of a screw cooperate with projections on a plate as a ratchet system to prevent rotation to effect installation but prevent counter-rotation between the plate and screw. Projecting structures on the underside of the plate engage the roofing material to prevent rotation of the plate relative to the roof.
A similar system employing a ratchet mechanism to prevent the screw from backing out is also described in Giannuzzi U.S. Pat. No. 4,763,456, issued Aug. 16, 1988. However, these various ratchet structures tend to give the fastener assembly an undesirably high profile, and may be susceptible to disengagement due to, among other things, failure of the ratchet members, due to, e.g., overtightening or undertightening during installation.
Other such systems employ a threaded connection between the plate and fastener. For example, DeCaro U.S. Pat. No. 4,361,997, issued Oct. 25, 1988 describes a fastener with upper and lower sets of threads with an intervening unthreaded area which cooperates with a stress plate bearing anti-rotation structure on its underside. The lower set of threads are threaded through the plate at the job site with the use of a special tool prior to installation. The upper threads engage the plate after the screw is substantially turned into the roofing deck. The anti-rotation structures engage the roof membrane and prevent the plate from turning.
A number of other mechanical systems have been proposed for preventing separation of the screw from the plate in such fasteners. Systems have been proposed which employ a cap over the head of the fastener (see, e.g., Verble U.S. Pat. No. 4,658,558, issued Apr. 12, 1987; Frankovitch U.S. Pat. No. 4,520,606, issued Jun. 4, 1985; Beneze U.S. Pat. No. 4,620,402, issued Nov. 4, 1986) or resilient spring mechanisms to maintain tension (see, e.g., Hewison U.S. Pat. No. 4,616,455, issued Oct. 14, 1986), or a nut or similar element disposed on the lower end of the screw beneath the rooftop to hold the fastener in place, (see, Sargent U.S. Pat. No. 4,727,699, issued Mar. 1, 1988). Application of a bonding or sealing agent over the head of a fastener, between a stress plate and the membrane, or both, has also been proposed. See, Sandquist U.S. Pat. No. 4,074,501, issued Feb. 21, 1978, and Frankovitch U.S. Pat. Nos. 4,455,804 and 4,467,581, issued Jun. 26, 1984 and Aug. 28, 1984, respectively. These fasteners are only partly effective in preventing the fastener from backing out, and require additional structure for that purpose.
Roofing fastener systems with provisions for preventing the head of the screw from protruding beyond the top of the plate, e.g., in the event of loss of installation tension, have also been proposed. For example, such a system, wherein the washer includes a flexible ring about the aperture that receives the screw, is described in Dewey U.S. Pat. No. 4,380,413 issued Apr. 19, 1983. Another such system employing a plastic washer having a resilient rib which engages the screw head to hold it down is described in Hasan U.S. Pat. Nos. 4,712,959, issued Dec. 15, 1987, and 4,757,661, issued Jul. 19, 1988.
However, all such roofing fasteners, which, in effect, rely on engagement between a stress plate and roofing membrane to prevent backout, are susceptible to a greater or lesser degree to failure due to loss of preload, i.e., compressive force between the fastener and roofing material holding the plate against the membrane. Loss of preload can occur when the underlying insulation deteriorates and shrinks due to, e.g., harsh weather conditions, vibrations, or the like. For example, in some instances, a rocking motion of the stress plate may be caused by wind load or roof traffic tending to collapse the insulation in the vicinity of the stress plate. Likewise, aging, temperature fluctuation, and moisture may also tend to collapse insulation.
When the insulation collapses, the clamping force applied by the stress plate to the membrane and insulation surface decreases from the level at which the fastener was originally installed preload. If the clamping force drops below a certain level, the stress plate ceases to engage the membrane, and concomitantly no longer prevents backout of the screw, or counters pullout of the membrane due to lateral forces.
Perhaps the most widely used roofing anchor is a plastic auger type fastener. In many instances, it is desirable for such a fastener to manifest a particularly aggressive thread pitch. Aggressive thread pitch tends to facilitate installation, and provides for better engagement between the auger fastener and the deck in certain types of deck materials. However, the use of the aggressive thread pitch makes such fastener particularly susceptible to loss of preload due to backout. One revolution of reverse rotation may cause as much as 3/8 inch backout. Thus, if for any reason the auger screw loses engagement with its cooperating stress plate, or, the stress plate loses engagement with the membrane, results can be disastrous.
Fasteners including a screw and a spiked stress plate which form a bond between on outer surface of the screw and an inner surface of the plate during installation are also known. Such fastener is described in U.S. Pat. No. 4,987,714, issued on Jan. 29, 1991 to Stuart Lemke. The screw and plate can be made of commonly available thermoplastic resins, and do not require special structures on the screw or plate to prevent the screw from unscrewing (backing out). The cleats on the bottom of the plate facilitate bonding through spin welding, and also help prevent disengagement of the plate from the membrane or the membrane tearing loose or fluttering.
The bond between the screw and the plate also tends to militate against rocking motion of the stress plate that might collapse the underline insulation. However, engagement between the stress plate and membrane is relied upon to prevent backout. Thus, while considerably more tolerant than other forms of roofing fasteners, the spin welded fastener continues to be susceptible to disfunction in the event that clamping force decreases sufficiently from the preload (installation) level.
A plastic auger type roofing fastener for use in such non-penetrating situations, which includes extendable wire barbs to engage insulation or other roofing material such as tectum is described in U.S. Pat. No. 4,655,659, issued Apr. 7, 1987 to Stuart Lemke. The wires are initially stored in an internal compartment within the screw body underlying a slidable plunger. To extrude the barbs, the plunger is forced downwardly by the ram of an impact type installation tool. Another such fastener is described in U.S. Pat. No. 4,507,991, issued Apr. 2, 1985 to Stuart Lemke, in which the internal chamber is threaded, and the plunger threadedly engaged therein. The plunger is then rotated to effect longitudinal motion to extrude the barbs.
The tools for installing such roofing fasteners are also known. An example is described in the aforementioned U.S. Pat. No. 4,507,991. Installation tools impart rotational motion to the fastener to screw the auger into the deck, then provide either an impact to the plunger within the fastener or a rotational motion to the plunger to cause the barbs to extend. However, such installation tools tend to be complex and expensive. In addition, impact installation tools tend to disrupt the integrity of low density decks, and denigrate the thread engagement between the fastener and decking material. Further, such impact installation tools are unsuitable for use with fasteners in which a bond is formed between screw and stress plate; the impact employed in setting the wire barbs disrupt the bond.